Rotary joints are well known and an extensive description thereof may be found in Chapter 7 of Volume 9 of the MIT (Massachusetts Institute of Technology) collection "Microwave transmission circuits". However, these rotary joints are not very well adapted to modern radar equipment where, for proper operating conditions, it is necessary to have, between the antenna, on the one hand, and the receiver and transmitter, a relatively high number of separate channels, 4 to 5 in number at least. This is particularly the case in a primary radar associated with a secondary radar whose antenna is mounted on the antenna of the primary radar and where the primary or principal radar must be provided with a transmission channel, a sum reception channel and at least one difference reception channel, whereas the secondary radar must have an interrogation channel, possibly a control channel for suppressing secondary lobes, a sum response channel, a channel for suppressing secondary lobes and a difference channel for establishing the null position.
With a prior-art rotary joint of the "Door Knob" type, for example, it is not possible to have more than three coaxial channels. This limitation comes from the external diameter of the joint which must be less than a wavelength. For a wavelength of the order of 30 cm, a diameter less than .lambda./.pi. is of the order of 10 cm.
With a rotary joint of another prior-art type, in which ancillary channels are in the form of rings spread out around the main channel, the number of channels is again limited for reasons of weight and mechanical complexity.
FIG. 1 shows very schematically a conventional rotary joint for a radar comprising, besides a main axial channel 1 with axis of rotation 2, another channel 3 which may be annular. This channel is formed by two identical rings 4, 5, ring 4 being fixed and forming an input while ring 5 is mobile and forms an output. Each ring comprises a so-called coupling disk 6, 7 connected to the input or output terminal by means of a respective distributor 8, 9. At 10 is shown a trap required for this type of joint. The whole assembly is mounted in a casing 11.
FIG. 2 shows schematically in horizontal section one of the rings of FIG. 1, specifically ring 5. As can be seen, the ring is engaged by the associated distributor 9 at four equiphase points a, b, c, d. It is attempted thereby to obtain a substantially uniform flow of the currents without appreciable fluctuation. However, for mechanical reasons, the length of the rings is not small compared with the wavelength. The phase of the HF signal therefore is not constant at all points of the ring. The rotation of one of the rings in relation to the other causes amplitude variations so that neither the amplitude nor the phase of the transmitted signals remains constant. It is necessary to provide traps which are difficult to realize and cause leaks and create spurious couplings between channels. This, added to the fact that the annular channels present a certain mechanical complexity and that the height of the main channel is limited, prevents a joint of this type from being able to accommodate a large number of channels.